In conclusion, we have investigated the implications of a possibly large
neutrino magnetic moment, up to the scale reported by XENON1T, on the helicities
of relic neutrinos as they propagate through the cosmic and galactic magnetic fields,
and found significant helicity modifications even if their magnetic moment is two orders of magnitude smaller than that suggested
by the XENON1T result. The present estimates of neutrino spin rotation can be sharpened by using detailed maps as well as numerical simulations of the astrophysical magnetic fields, e.g., [1]}, [2]}, [3]}, [4]}. We note, in addition, that the spin rotation of MeV energy neutrinos from the diffuse
supernova background [5]} as well as from neutron stars [6]} is also potentially detectable, although using different experimental techniques than for relic neutrinos (e.g., with the Gd-doped
Super-K detector and the inverse beta decay reaction) [7]}.
